It is generally necessary to isolate local telecommunication equipment from the line. This is generally achieved by means of a transformer and local circuit components to provide the necessary line matching characteristics although the specific components on the line side and equipment side need to be chosen to suit the characteristics and requirements of each country. However, regardless of the specific choice of components, arrangements give rise to unwanted harmonic distortion and intermodulation products. It is desirable to reduce such unwanted products.
The demands of telecommunication development are towards greater miniaturisation. However, the scope for size reduction of transformers is limited if the shunt inductance is to be sufficiently high to permit line matching, and if the winding resistance is to be kept acceptably low.
Known circuit arrangements also suffer from a frequency response which is not flat for transmission and reception if the line has a complex impedance causing attenuation distortion.
It is an object of the present invention to seek to optimise the performance of the coupling of equipment to a telecommunication line. This may be achieved by means of optimisation of a circuit arrangement in which the isolation function is separated from the line matching requirement.
A circuit layout resembling one of the preferred embodiments of the invention and containing two transformers has been known for at least ten years. However, such circuits have been used solely to secure good transhybrid loss performance, it not having been realised that the arrangement could be optimised so as to produce a very high standard of performance in respect of transhybrid loss, matching and distortion of all kinds simultaneously.
It has been found in embodiments of the invention that:
(a) although the flux density arising from the transmission signal is greater than in known conventional arrangements, the low source impedance minimises the distortion products arising from the transmission signal; PA1 (b) as the flux density due to the received signal from the line is small, so all intermodulation between the transmit and receive signal and distortion products arising from the receive signal in the line are very small; PA1 (c) whatever unwanted products are produced in the transmit line isolation transformer they are reduced by the transhybrid loss of the hybrid; PA1 (d) a high inductance transmit line isolation transformer is no longer required to achieve satisfactory match of the line; PA1 (f) as the matching network is on the line side of the line isolation transformers it is possible to use a common coupling unit for all countries and to use a preset impedance built into the line cord or selected by means in the line cord of local equipment to provide the only necessary line matching components for any particular line impedance; PA1 (g) when the winding resistance and leakage inductance of the transmit coupling transformer are negligibly small, the optimisation of the reference impedance to obtain optimum return loss also produces optimum transhybrid loss; PA1 (h) if desired, the transhybrid loss can be adjusted independently of the return loss; PA1 (i) by using an amplifier to provide feedback a low source impedance is possible; PA1 (j) it is possible to select component values and so create a duplex communication coupler system with zero voltage insertion loss on transmit and receive.